In order to suppress a generation of soot which provides a problem in a direct injection diesel engine, a technique in which an after-injection in a relatively short period of time is performed immediately after a main injection and soot generated in association with a combustion of the main injection is combusted together with fuel by the after-injection is described in Japanese Patent Application First Publication (tokkai) No. 2005-233163 and Japanese Patent Application First Publication (tokkai) No. 2000-227061 and so forth.
In such a technique of the after-injection as described above, a certain optimum value corresponding to an individual driving condition is present as an interval from a time at which the main injection is ended to a time at which the after-injection is started. As shown in a characteristic a of FIG. 8, as the interval shown in a lateral axis of FIG. 8 becomes gradually larger, a reduction of soot is observed when the after-injection at an appropriate interval is performed. It should be noted that a left end of the lateral axis of FIG. 8 indicates a case where the interval is zero, namely, a case where no after-injection is performed (in other words, a case where the injection is performed without split of the injection into the main injection and the after-injection). Hence, an injection timing of the after-injection in accordance with the driving condition (load, revolution speed, and so forth) is, for example, set in a form of a map. In the example of FIG. 8, such an optimum injection timing of the after-injection as shown in FIG. 8 is given as an injection timing IT1.
However, during a transient time of the engine, the reduction effect of soot according to the after-injection is lowered. For example, a characteristic b of FIG. 8 indicates an example of the characteristic of the after-injection during an acceleration time. As shown in FIG. 8, soot is hardly reduced at injection timing IT1 preset as an optimum point. When the interval is made larger than injection timing IT1, the reduction of soot is observed.
It can be thought that, according to the present inventor's research, since a fuel pressure (so-called, a rail pressure) when the main injection is carried out before the after-injection is lower than a target fuel pressure (a target fuel pressure during a steady state time) due to a response delay, during an acceleration time, it can be thought that this is due to not obtaining an expected gas flow within a cavity. That is, since the fuel pressure supplied to the fuel injection nozzle via the common rail is variably set on a basis of an engine revolution speed and a load, the target value of the fuel pressure is varied during a transient time and the target fuel pressure becomes, in general, higher during the acceleration time. However, since the response delay is present in the variation of the actual fuel pressure, the actual fuel pressure does not reach to an expected fuel pressure during the main injection and a momentum of a spray is reduced. Therefore, the injection timing of the after-injection which is optimum for the reduction of soot is made different.
FIGS. 9(a)-(c) are explanatory views for explaining an influence of gas flow within the cavity according to the main injection on the after-injection and show a local excess air ratio distribution within a combustion chamber in a form of contour lines. It should be noted that FIGS. 9(a)-(c) depict the excess air ratio divided into 14-stage levels. As a representative, a region denoted by a reference sign of E1 is a highest excess air ratio region, a region denoted by a reference sign of E3 is a lowest excess air ratio region, and a region denoted by a reference sign of E2 is a middle excess air ratio region. FIG. 9 (a) shows an excess air ratio distribution within the combustion chamber when the after-injection is carried out (at a time of the end of the after-injection) after the main injection under an appropriate fuel pressure is carried out. At this stage, a reverse squish flow is generated due to a lowering start of a piston and the momentum of the main injection. Riding on this squish flow, a combustion section surrounding of a bottom section of the cavity is tried to be moved toward a center part of the cavity. In FIG. 9 (a), since expected gas flow is obtained, spray F of the after-injection is injected toward a region in which oxygen is present to some degree. Hence, the reduction of soot is achieved.
Whereas, FIG. 9(b) shows a state in which the fuel pressure of the main injection is low. In this case, since the momentum of the main injection is low and the reverse squish flow is weak, the after-injection is carried out before the combustion section surrounding the bottom section of the cavity is moved toward the center section of the cavity. Therefore, spray F of the after-injection is injected toward a region in which oxygen (quantity) is small so that an increase of soot is introduced.
FIG. 9(c) shows a state of a case in which the after-injection is executed at a crank angle slightly retarded than a case of FIG. 9(b). Since the combustion section is moved toward the center section of the cavity, spray F of the after-injection can be given to the region in which oxygen is present to some degree and the reduction of soot can be achieved.
Even during a deceleration, from the similar reason, the appropriate injection timing of the after-injection is made different from a case during the steady state. Since, during the deceleration, in general, the fuel pressure becomes excessively higher than the target value due to a response delay, the appropriate injection timing of the after-injection becomes furthermore earlier.